Numerous applications, including but not limited to those using certain microprocessors, digital signal processors, field programmable gate arrays and programmable logic devices, have requirements concerning the relative behavior between multiple power supplies. The applications may require that power supplies ramp up and down together at a specific rate (see, e.g., FIG. 2A). The applications also may require that the supplies ramp up and down with other behaviors, such as in the case of supply sequencing (see, e.g., FIG. 2C).
A common method used to control the behavior of multiple supplies during power up and power down is to use series FETs (field effect transistors). The drain of the FET is connected to the power supply's output, and the gate node voltage of the FET usually is slewed at a predefined rate which sets the ramp up and ramp down rate of the controlled supply (see, e.g., FIGS. 1A-B). This is the method used by a product manufactured and sold by Linear Technology Corporation, the LTC1645, Dual-Channel Hot Swap Controller/Power Sequencer.
One disadvantage of series FETs is that the current flowing through the FET produces a voltage drop that appears between the input power supply (at the drain of the FET) and the voltage at the load (which may be connected to the source of the FET). Additionally, if the requirements of the system are stringent, a feedback loop is necessary to maintain a predefined relationship between multiple supplies due to the mismatch between the FETs. This type of feedback loop is used by a product marketed by Summit Microelectronics, Inc., the SMT4004, Quad Trakking™ Power Supply Manager.
Another method places a shunt FET between two supplies. When the voltage of the lower supply is too low, current flows through the FET from the upper supply and pulls up the lower supply. If the voltage of the lower supply is higher than that of the upper supply, the FET is turned on, shorting the upper and lower supplies.
Power supplies often are capable of sinking current and sometimes regulate to a low voltage as they are powered up. In the shunt FET configuration, this poses serious hazards. If the lower supply is capable of sinking current, it may sink a large amount of current from the upper supply and damage the shunt FET. Similarly, the upper supply may sink current when it falls below the lower supply. When the FET shorts the supplies, it also may overload an input power supply, causing its voltage to collapse, or the large current potentially dumped to ground could cause ground bounce. Furthermore, this method only clamps the two supplies together. It does not allow sequencing or a controlled ramp up and ramp down behavior. This is the method employed by a product marketed by Maxim Integrated Products, the MAX5039/MAX5040, Voltage-Tracking Controllers for PowerPC, DSPs, and ASICs.
In view of the foregoing, it would be desirable to be able to provide methods and circuits for tracking or sequencing multiple power supplies in defined relationships.
It also would be desirable to be able to provide methods and circuits for tracking or sequencing multiple power supplies in defined relationships that are user-programmable.
It further would be desirable to be able to provide methods and circuits for tracking or sequencing multiple power supplies that present a high impedance to one or more of the power supplies.